As an example of spline shafts, there is a propeller shaft that transmits the driving force of powder unit, which is boarded on vehicle, to wheel. This propeller shaft is placed between transmission, which is connected to power unit, and differential gear, which is connected to wheel, thereby transmitting the output of the transmission to the differential gear. Here, depending on shocks from road surface and vehicle's driving conditions, and the like, the relative positional relationship between the transmission and the differential gear moves. In particular, in order to absorb the relative position's movement in the front/rear direction of vehicle, the propeller shaft is divided into a first shaft, which is provided with an outer-peripheral spline, and a second shaft, which is provided with an inner-peripheral spline (see Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-359,087). That is, it is configured so that the outer-peripheral spline and the inner-peripheral spline can slide axially (in vehicle's front/rear direction).